Hyponatremia: Evolving diagnostics and emerging therapeutics in clinical practice

Figure 3 Furst equation and fluid restriction strategy in euvolemic hyponatremia. The Furst equation provides a practical estimation of urine tonicity relative to plasma sodium concentration and helps anticipate the likelihood of response to fluid restriction (FR). A value < 0.5 indicates maximally dilute urine and high probability of correction with standard FR (approximately 800-1000 mL/day). Intermediate values (0.5-1.0) suggest partial concentration and warrant a stricter trial of FR (approximately 500 mL/day) under close monitoring. Ratios > 1.0 reflect concentrated urine (surrogate for urine osmolality > 100 mOsm/kg), indicating poor response to FR and the need for early consideration of second-line therapies (urea or vasopressin receptor antagonists). U_na: Urinary sodium concentration; U_k: Urinary potassium concentration; FR: Fluid restriction.